1. Field of the Invention
The present invention generally relates to a nonlinear inductor with magnetic field reduction for use in a raster-scanned CRT display such as a television receiver or a computer visual display unit.
2. Description of the Related Art
A raster-scanned CRT display comprises a line timebase circuit for generating a sawtooth deflection current in a pair of electromagnetic line scan coils. The deflection current energizes the coils to produce a time-variant magnetic deflection field. The deflection field scans one or more electron beams from one side of the CRT screen to the other side during a line scan period and rapidly returns the electron beam during a retrace or flyback period to the start of the next line of the raster to be scanned. The line timebase circuit typically comprises an inductor connected in series with a high-voltage solid-state switch such as a bipolar transistor. During the line scan period, the switch is closed and current flows through the inductor and switch from a high voltage DC supply (typically 100 V) to ground. The current flowing through the inductor and the current flowing in the scan coils increase at a rate proportional to the voltage across the inductor. During the flyback period, the switch is opened. The current in the inductor rapidly reverses and a back Electromotive Force (EMF) or "flyback pulse" is generated across the inductor by the collapsing magnetic field in the inductor. The flyback pulse is controlled by a tuning capacitor connected across the switch. The back EMF dissipates causing the current in the scan coils to rapidly reverse. The electron beam is thus deflected rapidly back to the start of the next scan line.
Ideally, the deflection coils would be pure inductances. In practice however, the coils have a DC resistance. The resistance causes an asymmetric nonlinearity in the sawtooth deflection current. The nonlinearity gives rise to objectionable asymmetric distortion of an image displayed on the CRT screen. The asymmetric distortion is a function of the inductance-to-resistance ratio of the deflection coils and therefore varies proportionally with frequency.
In a typical CRT display, the asymmetric nonlinearity is corrected by connecting a nonlinear inductor in series with the deflection coils. The nonlinear inductor typically comprises a permanent magnet positioned adjacent an inductive winding. The inductance of the nonlinear inductor varies about an operating point as a function of the current flowing through the winding. Some nonlinear inductors comprise a movable magnet which can be set in position during a manufacturing process step. The movable magnet permits manual adjustment of the polarity of the magnet relative to the winding, and therefore changes the operating point of the inductor.
In a color CRT display, low-level magnetic correction fields are arranged around the neck of the CRT to optimize electron beam convergence and color purity. The magnetic field from the magnet can interfere with these correction fields and thus degrade picture quality. To minimize such degradation, the not, linear inductor Is usually located as far away from the CRT as possible. However, because CRT displays are becoming smaller in size, and CRT resolution is increasing, it is becoming increasingly difficult to prevent the stray field from the non linear inductor from interfering with the correction fields.
Such interference may be prevented by magnetically screening the nonlinear inductor from the CRT. However, this is both costly and space consuming. Alternatively, as described later with reference to FIG. 7 of the accompanying drawings, another magnet may be located in the vicinity of the nonlinear inductor to deflect the magnetic field from the nonlinear inductor away from the CRT. However, the deflection provided by the other magnet is very sensitive to its position relative to the CRT and the nonlinear inductor. Therefore, printed circuit board space must be set aside for accommodating the other magnet.